The pressure-enthalpy diagram (log P/h diagram) is a very useful tool for refrigerant technicians. First, an explanation of how the diagram is built up is given, and then its use is described.
Figure 2.1 shows the principle of a log P/h diagram, and indicates the refrigerant's various thermodynamic states. This diagram can be seen as a map of the refrigerant. The area above and to the left of the saturation line for liquid (A-CP in Figure 2.1) is the area where the refrigerant is sub-cooled, i.e. the temperature is lower than the saturation temperature for the pressure range in question. The area above and to the right of the saturation line for gas (CP-B in Figure 2.1) is the area where the gas is superheated, or overheated, i.e. the gas has a higher temperature than the saturation temperature at that pressure. The area below the saturation lines for liquid and gas (A-CP-B in Figure 2.1) represents the conditions where the refrigerant can change its state of aggregation from liquid to gas or vice versa. Hence, there is a mixture of gas and liquid.
The practical meaning of the critical point (CP) is that at temperatures higher than this, the refrigerant cannot be condensed, no matter how high the pressure. Therefore, compression refrigeration systems normally operate at temperatures below the critical one.
Lines of constant temperature (isotherms) are vertical in the sub-cooled liquid region, horizontal (i.e. parallel to the constant pressure lines) in the liquid + vapor mixture region, and drop steeply towards the enthalpy axis in the superheated gas region (see Figure 2.2). The constant pressure lines (isobars) are parallel to the x-axis.